This invention relates to transducers that convert mechanical energy into acoustical energy. More specifically, it relates in one form to a loudspeaker with a piezoelectric actuator and in another form to a flat film speaker compatible with a video display.
All acoustic transducers must supply the atmosphere with an alternating positive and negative pressure. In its simplest form a linear motor, whether electromagnetic, electrostatic or piezoelectric, actuates a diaphragm that is sometimes part of the motor itself.
The overwhelming majority of loudspeakers are electromagnetic transducers. Referred to as dynamic loudspeakers, this class has essentially remained unchanged since the 1920""s. Electromagnetic motors have long linear travel. This attribute is used to move a relatively small rigid diaphragm (in the manner of a piston, or xe2x80x9cpistonicxe2x80x9d as the term is used in the loudspeaker art) over the long excursions needed for acoustic use. The tradeoff is the low efficiency of this action at a distance.
Electrostatic and piezo devices have a much higher electrical-to-mechanical coupling efficiency than dynamic loudspeakers. They have been used to a limited degree for many decades, but their theoretical high efficiency has been limited by their comparatively short linear travel. In the case of electrostatics, very large diaphragm structures, several feet long on each side, are needed to generate the required acoustic displacementxe2x80x94or they are simply built small enough to be of practical size, but limited to operation in the upper frequencies where long excursions are not needed. Piezoelectrics have the highest theoretical efficiency of all, but they have been relegated to the upper frequencies exclusively because of their comparatively small size and limited excursion.
It is therefore an object of this invention to provide a new class of mechanical-to-acoustical transducers, especially loudspeakers, that can employ any of the aforementioned actuators, but are particularly well suited to transforming the high efficiency, short linear travel of a piezo motor into a high-excursion, pistonic-equivalent diaphragm movement.
Another object of this invention is to provide a flat, film-type speaker for televisions, computer monitors, or the like where the display is viewed through the speaker.
A mechanical-to-acoustical transducer according to the present invention has at least one actuator, preferably a piezo motor, coupled to a thin, rigid, yet flexible, diaphragm that is anchored at a location spaced from the point or points of coupling of the diaphragm to the actuator. The diaphragm is curved when viewed in vertical section between the point of the actuator coupling and the anchoring point or points. The diaphragm is formed of a thin, flexible sheet material. For screen-speaker applications, it is formed of a material that is transparent as well.
In one form, the actuator is located at or near a vertical centerline that divides the diaphragm into two sections (in effect providing two transducers). The lateral edges of the diaphragm distal from the actuator are fixed at both edges to anchor them against movement. The fixed edges can be secured to a frame that supports the diaphragm and a piezo bimorph drive. A gasket secured at the edges of the diaphragm helps to maintain the pressure gradient of the system. The two diaphragm sections each have a slight parabolic curvature viewed in a plane through the diaphragm, and orthogonal to the vertical axis. One section is curved convexly and the other concavely in an overall xe2x80x9cSxe2x80x9d shape when the piezo bimorph is in a centered, rest position. A DC potential can be used to minimize hysteresis that is present in piezo structures. Hysteresis is also present in the linear magnetic motors commonly used in the typical loudspeaker, but this hysteresis cannot be countered actively as it can with a biomorph. With the actuator at the midpoint of the xe2x80x9cSxe2x80x9d curve, positive and negative diaphragm displacement asymmetries cancel out, yielding a substantially linear net diaphragm excursion in response to an essentially linear lateral excursion of the drive.
The actuators useful in loudspeaker applications are characterized by a high force and a short excursion. The diaphragm is characterized by a large, pistonic-equivalent excursion. A typical amplification, or mechanical leveraging, of the excursion is five to seven fold. Multiple actuators arrayed end-to-end can drive different vertically arrayed portions of the diaphragm. In another form, the actuator is secured to one lateral edge of the diaphragm.
In another form, the invention uses a diaphragm that is a thin sheet of a rigid transparent material secured over a video display screen of a television, computer monitor, or the like. In a preferred form, the sheet is mechanically pinned and/or adhesively bonded along or near its vertical centerline (preferably at its top and bottom edges) to create two lateral sections, or xe2x80x9cwingsxe2x80x9d, each with three free edges, upper, lower and lateral. Linear actuators are operatively coupled to the free lateral edges of both wings, preferably by adhesive bonding with the diaphragm edge abutting a free end of the actuator generally at right angles. A lateral linear motion of each actuator then causes an increase or decrease in a slight curvature of an associated wing. The curvature is preferably that of a parabola (viewed in a plane orthogonal to a vertical axis, e.g., the pinned centerline). For typical video displays it has a xe2x80x9cradiusxe2x80x9d of about one meter (xe2x80x9cradiusxe2x80x9d assuming that the parabola is closely approximated by a circle of the radius).
The actuators are electromechanical, such as electromagnetic, piezoelectric, or electrostatic. Piezo actuators do not create a magnetic field that interferes with the display image and are preferred. For loudspeaker applications, the actuators are typically high-force, short-excursion types. The speaker of this invention converts this movement actuator into a low-pressure, amplified-excursion diaphragm movement. The sheet may have a layer of a polarizing material bonded to it to control screen glare, or utilize other known treatments that are either applied or molded onto the surface of the diaphragm to produce optical effects such as glare reduction.
These and other features and objects of this invention will be more readily understood from the following detailed description that should be read in light of the accompanying drawings.